Using Kodak PhotoCD Technology for Preservation and Access:
A Guide for Librarians, Archivists, and Curators
Anne R. Kenney and Oya Y RiegerDepartment of Preservation and Conservation, Cornell
University Library for
New York State Education Department, Program for the Conservation and
Preservation of Library Material

FORM 7

Don Brown's Recommendations for Kodak
Photo CD Scanning Process

First, there are some very basic scanning practices all Photo CD
service providers should follow. I would like to think these do not
need to be enumerated by customers negotiating with a Photo CD
scanning service, however I will go ahead and list them:

Regular scanner maintenance.

Clean environment.

Current scanner calibration.

Proper warm-up time (30 minutes) is observed after scanner is
turned on, and before scanner corrections are performed.

Clean film (clean film of dust before scanning).

I also feel the need to add a few words about different Photo CD
scanners. The system supports two scanners: the PCD Film Scanner
2000, which is a 35mm-only scanner, and a multi-format professional
scanner, the Professional PCD Film Scanner 4050. The predecessor to
the 4050 scanner is the 4045 scanner, and it is still used by some
labs. The 4050 scanner has improved internal electronics resulting in
better dynamic range. An upgraded 4045 scanner is called a 4045
Enhanced scanner and is equivalent to the 4050. For scans of
negatives, the improved dynamic range of the 4050 is not needed, so
there is very little difference between negative film scans on the
2000 and 4050 scanners. The primary benefit of the 4050 is in
scanning chromes. One additional benefit of the professional scanners
is that higher resolution scanning is possible. These scanners can
produce Image Pacs with a maximum resolution of 64BASE (6K x 4K). For
a scan of 35 mm images, a 64BASE Image Pac really does not provide
any more image detail than a normal 16BASE (3K x 2K) Image Pac. The
benefit of higher resolution is appreciated more in scans of mid and
large format films. So, the conclusion is that very good results can
be achieved with scans of 35 mm negative film on the 2000 scanner. As
a happy coincidence, this is also the most cost effective option due
to the productivity of the 2000 scanner.

Now, when we did our scanning, we tried to assure the Kodak Gray
Scale was captured correctly. This is based on the belief I've
mentioned before: if you get the neutrals correct, most of your work
is done. So, a technique was derived to do a first-approximation
calibration to the Kodak Gray Scale to improve the quality and
consistency of the scans. The technique assumes the scale is
perfectly neutral, the copystand lighting is perfectly uniform, and
the frame to frame exposure variability of the film is negligible.
All of these are reasonable assumptions if care was taken during the
photography. Scan to scan variability of the scanner is known to be
very small.

Individual scans can be custom-adjusted with this same technique,
but the desire is to create a standard correction for all images at
the beginning of the scanning session and automatically apply it to
all subsequent images. The PIW software accommodates such a workflow,
and after the initial setup, the scanning process can be very
efficient.

The technique consists of three basic steps:

Create aim Photo YCC values for the Kodak Gray Scale steps.

Disable the auto-balance feature on the PIW.

Determine color and tone adjustments for the scanner that then
are applied to subsequent scans.

1. Create Aim Photo YCC Values

The assumption was made that the Kodak Gray Scale is perfectly
neutral throughout the range, not a bad supposition, and that its
location in an image is representative of the lighting in the rest of
the image. Given that, aim Photo YCC values can be calculated.

Photo YCC is a luminance-chrominance color space, and therefore a
perfect neutral will have no chrominance, or color, component. Photo
YCC encodes this neutral point with chrominance values of C1=156 and
C2=137. The luminance, or Y channel, records the variation from black
to white, low numbers dark and high numbers light.

Given the assumptions above, the C1 and C2 values are set to 156
and 137 respectively for all the gray scale steps. The Y values can
be calculated from the reflection density of the patches of the
actual Kodak Gray Scale. Ideally, the same scale used in the
photography should be measured. The measurement is made on a
reflection densitometer, reading visual density. (In our case, we
measured one of our Kodak Gray Scales and made a leap of faith that
it was identical to the one on the image.)

The conversion from reflection density to Y
luminance value is:

Y =
181.9(1.099*10-0.45D -
0.099)

where D
= reflection density

It should be noted that this conversion is only valid for perfect
neutrals, and not for colors.

The Kodak Gray Scale has nominal density values for each of the
patches, ranging from 0.0 to 1.9 in 0.1 increments. The literature
included with the scale states, "Density increments may vary slightly
above and below the stated 0.10 value. Neutrality and uniformity of
the patches are carefully controlled". With this in mind, it is
suggested the specific scale used in the photography be measured, and
aims derived from those measurements. In the interest of providing an
example, and if measurement is not possible, the Photo YCC aim values
for the nominal Kodak Gray Scale densities are given below.

Step
Number

Nominal
Density

Y
Value

C1
Value

C2
Value

A

0.0

182

156

137

1

0.1

162

156

137

2

0.2

144

156

137

3

0.3

128

156

137

4

0.4

114

156

137

5

0.5

101

156

137

6

0.6

89

156

137

M

0.7

79

156

137

8

0.8

69

156

137

9

0.9

61

156

137

10

1.0

53

156

137

11

1.1

46

156

137

12

1.2

40

156

137

13

1.3

34

156

137

14

1.4

29

156

137

15

1.5

24

156

137

B

1.6

20

156

137

17

1.7

16

156

137

18

1.8

13

156

137

19

1.9

10

156

137

2. Disable the Auto-balance

Color negative film scanned on PIW systems is automatically
analyzed for color and density. The analysis algorithm otherwise
known as the Scene Balance Algorithm or SBA, produces color balance
values which normally are applied to the scanned image. The operator
can then make additional manual adjustments based on the preview
image on the monitor.

Assuming each of the images were captured with identical lighting
and exposure, a single set of manual adjustments will correct for
each frame in the series. The auto-balance algorithm will only
confound this technique and therefore needs to be disabled.

The method to disable the algorithm is somewhat arcane. It
requires creating a new film term based on the normal film term for
the particular film type being scanned. Service providers are
familiar with creating custom film terms, and the process is well
documented in the PIW manuals. The esoteric part is the value which
must be changed within the film term to disable the auto-balance.
Without going into the details, the new film term must set CMD=3.
This step is critical, and not difficult to perform. It may, however,
be an unusual request.

The default film term for color transparencies has the
auto-balance disabled, so this step in not necessary for scans of
chromes.

3. Determine Color and Tone Adjustments

The image adjustments are derived during the preview part of the
scanning operation. A preview scan is made of the first image in the
series using the newly created film term. Manual color and tone
adjustments are made on this image to drive the sampled Photo YCC
values of the patches of the Kodak Gray Scale to the calculated aim
values.

Two sets of adjustment tools are available to the operator:
contrast and balance image adjustments, and the Tone Scale Adjustment
tool. Both are manipulating the same parameters, and both can achieve
the same results. For this type of adjustment, however, the Tone
Scale Adjustment tool is easier to use.

This is an iterative process, but the following guidelines should
help. The Tone Scale Adjustment tool allows the operator to pick a
highlight and a shadow point on the image, and set RGB aim values for
them. The highlight point should be on patch A or 1 on the Kodak Gray
Scale, and the shadow point on 15 or B. The RGB aims should be
adjusted, with the constraint that R = G = B to preserve neutrality,
until the Photo YCC aims for that patch are achieved. After the
highlight and shadow patch aims are attained, the Photo YCC values of
the rest of the steps in the scale should be sampled to make sure
they are neutral and near the correct aim values. Selecting different
highlight and shadow patches may improve the overall tone scale
neutrality. When the best compromise is determined, the adjustments
are held for the remaining scans.

This process is essentially a straight line adjustment to what may
be a nonlinear variation. In other words, there may be some color and
density variation within the tone scale that cannot be fully
corrected. If properly done, however, this technique can correct for
the majority of color and tone problems and give very good results.
The success of this process is somewhat dependent on the skill of the
operator, but the aim values make this task much more manageable.

Tone scale patches on a few subsequent images should be examined
to assure the image on which the corrections were based is
representative of the entire series. Once this confidence is
achieved, the rest of the scanning can be done with no operator
adjustments. If anything changes within the a series of images, such
as photographic lighting difference, film emulsion change, even film
processed at a different time, it is a good idea to repeat the
correction for each sub-series within the entire series of images.